Mobile data transmission and data services are constantly making progress, wherein such services provide various communication services, such as voice, video, packet data, messaging, broadcast, etc. In recent years, Long Term Evolution LTE™ has been specified, which uses the Evolved Universal Terrestrial Radio Access Network E-UTRAN as radio communication architecture according to 3GPP specification.
Further, 3GPP has been working on improving the dedicated channel DCH for more efficient circuit switched CS voice support, targeting Release 12 of the ‘Wideband Code-Division Multiple Access’ WCDMA and ‘Universal Mobile Telecommunications System’ UMTS specifications.
According to the respective specification of the year 1999, i.e. from the very introduction of WCDMA DCH, a continuous transmission is used, where 20 ms-long voice frames are transmitted over 20 ms Transmission Time Interval TTI over the air. Each active user has a dedicated code channel, and multiple users transmit and receive the voice frames in parallel.
WCDMA, ever since the release of the year 1999, features a compressed mode CM, which is used to generate transmission and reception gaps enabling the user equipment UE to perform inter-frequency and inter-radio access technology RAT measurements for mobility purposes. For employing the enhanced DCH, it needs to be able to provide measurement gaps as well. However, the typical CM gap pattern is not compatible with the uplink control channel structure selected for the enhanced DCH channel for voice, as it needs 10 slots to transmit the Transport Format Combination Indication TFCI information, but only 8 are available with the existing compressed mode with the gap pattern typically characteristic for conversational services such as voice.
In particular, as indicated above, the compressed mode CM according to 3GPP-release of 1999 specifications is designed to provide gaps in transmission that allow for the UE to retune its receiver on another frequency for a short time duration in order for it to perform measurements on that frequency. A typical CM parameterization for CS voice link is a 14-slot gap that spans over two radio frames and repeats once every 40 ms. Such structure is deemed sufficient for fast detection of inter-frequency neighbor WCDMA cells, or other radio systems, such as GSM or LTE, on another frequency.
The loss of slots due to the CM gap is compensated for by increasing the data rate and transmission power of the remaining slots in the radio frame experiencing the CM gap, so that the full voice frame is still transmitted over the air. Also the control channel slot formats are changed so that there are enough available control channel bits to transmit all needed information in the reduced number of slots being transmitted.
FIG. 1a schematically shows the transmission of a voice frame on WCDMA in the case of a normal (i.e. uncompressed transmission) according to the prior art. A radio frame of 10 ms comprises 15 slots, and a CS voice frame TTI is 20 ms. Further, FIG. 1b schematically shows transmission of a voice frame on WCDMA in the case of a compressed mode transmission according to the prior art. Thereby, a CM gap is provided, which typically comprises 14 slots. The CM gap starting at slot #9 (slots 1 . . . 8 transmitted, 9 . . . 15 not) is not compatible with the selected uplink control channel structure for enhanced DCH, where the first 10 slots are needed to deliver the TFCI information. Further the compressed mode structure according to the prior art is not very efficient compared to what can be achieved when the enhanced DCH properties were exploited.
Still further, FIG. 2 illustrates a proposal for CM with enhanced DCH according to 3GPP ‘R1-140253’. A 4-slot gap is generated in the middle, and after the downlink frame early termination acknowledgement bit (FET-ACK) is sent in the UL the remaining non-transmitted slots complement the gap. FIG. 2 also shows the uplink Dedicated Physical Control Channel DPCCH construction. The first 10 slots carry pilots (6 bits per slot), Transport Format Combination Indication TFCI (2 bits) and Transmit Power Control Command TPC (2 bits). The full TFCI information is encoded to these 10 slots, informing the Node B the transport format used on the DPDCH. The remaining slots substitute the TFCI with FET-ACK, first indicating NACK in each slot, until the DL voice frame successfully decodes, after which the UE starts to transmit ACK. However, this solution is cumbersome due to splitting the gap in two, and the guaranteed contiguous gap duration is very short (gap of 2.6 ms) and does not guarantee e.g. LTE measurements, where the Synchronization Channel SCH essential for detecting there is a cell repeats every 5 ms.